The invention relates to a method and system for inhibiting freezing of low salinity water in a subsea low salinity water injection flowline.
It is known from International patent applications WO 2012/051511, WO 2010/092097 and WO2010/092095 to Enhance Oil Recovery (EOR) by injecting desalinated water into an oil containing formation.
At offshore locations the low salinity water may be obtained by desalinating seawater at an offshore platform and then injecting the desalinated water flux through a subsea water injection flowline to a subsea wellhead of subsea desalinated water injection well.
In arctic and deepwater environments the ambient seawater temperature at the seabed may be −2 degrees Celsius. At this subzero temperature the seawater surrounding the subsea flowline does not freeze, but the desalinated injection water in the subsea injection flowline, which has a freezing point of about 0 degrees Celsius, may freeze and plug and rupture, in particular if the flux of low salinity water would temporarily stop, for example due to an injection process upset.
Upon melting, the flowline contents would spill into the sea and the flowline would require replacement causing production deferment and unplanned expenditure.
Known solutions to prevent freezing of subsea flowlines involve flowline heating, insulation and/or injection of anti-freeze additives, which are costly both installation-wise and operation-wise and require a active flowline monitoring and/or anti-freeze injection systems that are prone to malfunctioning. A potential platform black-out is one scenario where not only the desalinated water injection would stop, but where the heating and/or anti-freeze injection would stop as well.
There is a need for an improved method and system for inhibiting freezing of subsea desalinated water injection flowlines which are cost effective and less vulnerable to malfunctioning than available flowline heating, insulation and anti-freeze injection systems.